Anaerobic Digestion (AD) is a way of converting organic biomass to biogas. Hazardous chemical waste can also often be digested by microorganisms and disposed of by fermentation.
AD produces biogas, a mixture of primarily methane and carbon dioxide, which can be used for energy. This is an advantage over landfilling waste.
AD has three distinct phases: hydrolysis, acetogenesis and methanogenesis. Hydrolysis and acetogenesis often occur in the same tank and are thus sometimes considered one phase.
In AD, anaerobic microorganisms ferment carbon substrates to products in the absence of oxygen or oxygen surrogates. For instance, some organisms transform hexose sugars to ethanol and CO2. Other common fermentation products include lactic acid, acetic acid, butyric acid, H2, and methane. Many of these compounds are themselves substrates for further anaerobic metabolism by other microorganisms. However, two fermentation products cannot be further fermented—methane and CO2. Thus, all anaerobic decomposition can ultimately lead to methane and carbon dioxide.
AD biologically is generally considered to occur in two phases: (1) Breakdown of sugars and carbohydrates to smaller molecules, particularly organic acids such as acetic acid and butyric acid. This is known as hydrolysis or sometimes as acid production. And (2) production of methane and CO2 from the smaller organic molecules produced in phase (1), known as methanogenesis. Different organisms catalyze phases (1) and (2). Acid forming microorganisms and other microrganisms, that include both facultative and obligate anaerobic microorganisms catalyze phase (1), the hydrolysis phase. Organisms that produce methane are called methanogens. Methanogens can produce methane from acetate by the reaction acetate+H2O→methane+HCO3−. Methanogens can also produce methane from hydrogen and CO2 by the reaction 4H2+HCO3−+H+→CH4+3H2O. The hydrogen for methanogenesis from carbon dioxide in nature comes from fermentation of reduced carbon substrates. Only methanogens—which are obligate anaerobes—produce methane, hydrogen and carbon dioxide through the cleaving of acetate and formate and remove protons from the cytoplasm.
Methanogenesis is the most essential part of AD as it is the only stage that removes protons from the cytoplasm to allow the preceding steps to proceed. A well-functioning methanogenesis stage is key to maintaining a functional and efficient AD systems. Without it AD systems become septic and fail.
Of all metabolic pathways, methanogenesis yields the least amount of energy as the end product—methane—has a high enthalpy. Consequently there is minimal energy yield for the organisms involved causing them to grow very slowly. System design to retain biomass is critical. Traditional AD systems accomplish stability through large tanks that provide adequate time for a stable methanogen population to be maintained. This results in large tanks that are expensive to build and are not optimum.
Even in such tanks the process is always somewhat incomplete: some portion of the substrate is not digested all the way to methane and CO2. And the speed of the process is a limiting factor that determines the size of reactors needed. Thus, more efficient and faster methods of anaerobic digestion are needed, and digester systems that facilitate or allow more efficient and faster anaerobic digestion are needed.